Web Robots (also known as Web Wanderers, Crawlers, or Spiders), are programs that traverse the Web automatically. Search engines such as Google use them to index the web content, spammers use them to scan for email addresses, and they have many other uses.
Web site owners use the /robots.txt file to give instructions about their site to web robots; this is called The Robots Exclusion Protocol.
It works likes this: a robot wants to vists a Web site URL, say http://www.example.com/welcome.html. Before it does so, it firsts checks for http://www.example.com/robots.txt, and finds:
The "User-agent: *" means this section applies to all robots. The "Disallow: /" tells the robot that it should not visit any pages on the site.
There are two important considerations when using /robots.txt:
* robots can ignore your /robots.txt. Especially malware robots that scan the web for security vulnerabilities, and email address harvesters used by spammers will pay no attention.
* the /robots.txt file is a publicly available file. Anyone can see what sections of your server you don't want robots to use.
On this site http://www.robotstxt.org/ you can learn more about web robots.